Many mobile information devices such as laptop personal computers, cellular phones, etc., include a power saving mode for the purpose of saving battery power when the devices are in a standby state.
Specifically, in a conventional cellular phone, a battery power supply VBAT, utilizing 3.6 volts is transformed by a regulator 5 into a constant voltage Vcc having 2.0 volts, and is supplied to a CPU 2, which in turn is connected to an operation key 3 and a reception section 4 as shown in FIG. 6. In such a conventional configuration, when the operation key 3 is not activated for more than a prescribed time period, the CPU 2 switches an internal circuit to a low voltage operation condition in order to save power. The CPU 2 simultaneously outputs a voltage switching signal, under a power saving mode, to the regulator 5 so as to decrease a power output from the regulator 5.
The cellular phone is generally driven by a battery power supply VBAT. When an output of the battery power supply VBAT becomes lower than or equal to 2.0 volts, an output of the regulator 5 also becomes lower than or equal to 2.0 volts. Accordingly, a voltage detecting section is generally provided to continuously output reset signals to the CPU 2 so as to deactivate the CPU 2 in order to avoid operation when the output of the regulator 5 becomes less than or equal to a prescribed reference voltage (e.g., 1.9 volts). However, when the CPU 2 of the cellular phone enters into the power saving mode under the above-mentioned procedure, the voltage detecting section also detects such a decreased voltage and outputs a reset signal to the CPU 2. As a result, the CPU 2 is deactivated.